1. Field of the Invention
The present invention relates to a wavelength division multiplexing transmission system for separating wavelength division multiplexing signals, where signal lights with different signal bandwidths (e.g. bit rates) are wavelength-division multiplexed, according to the signal bandwidth (bit rate), and processing separated signals individually and the relay node (compensating node) thereof.
2. Description of the Related Art
Recently the capacity of Wavelength Division Multiplexing (WDM) transmission systems is expanding. Methods used for increasing capacity are increasing the number of wavelengths to be multiplexed and are increasing the transmission speed (bit rate) of signals with each wavelength. Concerning the bit rate, WDM transmission systems with 10 Gbit/s have already been commercialized, and research and development of optical transmission systems with 40 Gbit/s are on-going.
To install 40 Gbit/s WDM transmission systems, however, a partial upgrade must be considered in terms of installation cost and upgrade of in-service, such as switching the bit rate of a part of the wavelengths of a conventional 10 Gbit/s WDM transmission system from 10 Gbit/s to 40 Gbit/s, or adding 40 Gbit/s bit rate signal lights to an open channel of the conventional 10 Gbit/s WDM transmission system, rather than switching all of the wavelengths to be multiplexed to a 40 Gbit/s bit rate at one time. In other words, upgrading to a system where 10 Gbit/s and 40 Gbit/s are mixed is under consideration.
As the number of wavelengths to be multiplexed increases, the wavelength interval of signal lights becomes dense, and in the current 10 Gbit/s systems, systems of which the wavelength interval (frequency interval) of adjacent signal lights is 50 GHz have been commercialized, and the use of a 100 GHz wavelength interval (frequency interval) is under consideration for 40 Gbit/s systems.
For this increase in density, a method often used is multiplexing/demultiplexing lights in the 1:N channel optical multiplex/demultiplex module using an arrayed waveguide grating (AWG) filter or multi-layer film filter, and further multiplexing/demultiplexing these lights using an interleaver. As an index of this density, spectral efficiency, which indicates the bit rate per unit frequency, is used. In the case of a WDM transmission system of which the bit rate is 10 Gbit/s and the frequency interval is 50 GHz, the spectral efficiency is 0.2 bit/s/Hz (=10 Gbit/s÷50 GHz), and in the case of a WDM transmission system of which the bit rate is 40 Gbit/s and the frequency interval is 100 GHz, the spectral efficiency is 0.4 bit/s/Hz (=40 Gbit/s÷100 GHz).
The interleaver is an optical multiplexer/demultiplexer which has a function to demultiplex a signal group with a certain wavelength interval into an odd channel and even channel so as to create a signal group with a double wavelength interval, or to multiplex the odd channel and even channel so as to create a signal group with a ½ wavelength interval (e.g. Kito, et al: PLC filter synthesis theory and application to the interleaver filter, NTT R&D, Vol. 50, No. 4, pp. 281-287, 2001).
In a system before upgrade, that is a system which wavelength-division multiplexes and transmits 10 Gbit/s signal lights with a 50 GHz interval, if an arbitrary channel is changed from 10 Gbit/s to 40 Gbit/s, and these wavelength-division multiplexing signals (WDM) are multiplexed/demultiplexed by an ordinary interleaver with a 50 GHz/100 GHz interval, the transmission quality deteriorates. This is because the spectrum width (bandwidth) of 40 Gbit/s signal lights is wider than that of 10 Gbit/s signal lights (e.g. four times wider), so the 40 Gbit/s signal components leak into an adjacent channel (cross-talk), and the spectrum of the 40 Gbit/s signals is also restricted, depending on the band, by the interleaver. Therefore it is difficult to upgrade an arbitrary channel to 40 Gbit/s.
If the interleaver with a 100 GHz/200 GHz interval, which is used for 40 Gbit/s transmission, is used, cross-talk and transmission quality problems do not occur, but the spectral efficiency decreases to 0.25 bit/s/Hz, since 10 Gbit/s signal lights are also transmitted with a 100 GHz interval, which means that an upgrade has no effect.
Also upgrading an arbitrary channel to 40 Gbit/s requires installing devices for 40 Gbit/s (e.g. multiplexer/demultiplexer, interleaver, wavelength distribution compensator, polarization dispersion compensator) for all the channels anyway, including the 10 Gbit/s channels, regardless which channel is upgraded to 40 Gbit/s, so that sufficient signal quality is insured.
For these devices, the signal spectrum of 40 Gbit/s normally spreads to four times that of 10 Gbit/s, so the required specifications (e.g. transmission characteristics, distribution characteristics) are stricter in a device for 40 Gbit/s than in a device for 10 Gbit/s. Therefore the device for 40 Gbit/s requires higher performance and higher specifications, which increases cost.
For example, FIG. 34A shows the transmission characteristics of a multiplexer/demultiplexer for high-speed 40 Gbit/s, and a multiplexer/demultiplexer for low-speed 10 Gbit/s, and since the multiplexer/demultiplexer for high-speed requires a higher flatness and more vertical edges than the multiplexer/demultiplexer for low-speed, price becomes higher. FIG. 34B shows the Q factor deterioration with respect to the shift of the central wavelength when the 40 Gbit/s signal (high-speed signal) is input to both the multiplexer/demultiplexer for high-speed and the multiplexer/demultiplexer for low-speed, where the Q factor deterioration is smaller in the multiplexer/demultiplexer for high-speed than in the multiplexer/demultiplexer for low-speed. In other words, the multiplexer/demultiplexer for high-speed has high performance using high specifications to decrease the Q factor deterioration, so the device cost is higher in the multiplexer/demultiplexer for high-speed.
Therefore it is not desirable to install a device for 40 Gbit/s for all the channels in terms of cost, and using a 40 Gbit/s device for 10 Gbit/s signals exceeds specifications, which also generates an unnecessary cost.